Cumulative effects doubled the drought impacts for one-fifth of ecosystems exposed to recurrent droughts during Australia's Millennium Drought

Drought-induced vegetation declines have been reported across the globe and may have widespread implications for ecosystem composition, structure, and functions. Thus, it is critical to maximizing our understanding of how vegetation has responded to recent drought extremes. To date, most drought assessments have focused on the impact of a single drought event, emphasizing the importance of drought intensity for vegetation responses. However, drought timing, duration, and repeat exposure may all be important aspects of ecosystem response with the potential for non-linear effects. Cumulative effects are one such phenomenon, representing the additional decline due to repeated or prolonged exposure to drought, and indicating gradual loss of ecosystem resistance. This study quantifies the frequency and magnitude of cumulative effects among Australian ecosystems as they responded to the Millennium Drought. Three distinct biophysical variables derived from satellite remote sensing were analyzed, including fraction of photosynthetically absorbed radiation, photosynthetic vegetation cover, and canopy density derived from passive microwave data. While cumulative effects were detected in only 20% or less of the landscape exposed to repeat or long-duration drought, they approximately doubled drought impacts in the areas where they occurred. Cultivated lands and grasslands were the most susceptible to cumulative effects from recurrent droughts. Despite being relatively infrequent in forests and savannas, cumulative effects from repeated or long-duration drought caused larger additional declines in these ecosystems. Overall, our study demonstrates that the temporal sequence of drought events can strongly influence the magnitude of drought impacts, which provides important insights into modeling vegetation response to recurrent drought events